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Theoretical physicists working at a blackboard collaboration pod in the Beecroft building.
Credit: Jack Hobhouse

Gianmarco Spera

PDRA in Active Matter / Theoretical Biophysics

Sub department

  • Rudolf Peierls Centre for Theoretical Physics
gianmarco.spera@physics.ox.ac.uk
Rudolf Peierls Centre for Theoretical Physics, room 60.26
Personal Website
  • About
  • Publications

Fluctuation-induced first order transition to collective motion

Journal of Statistical Mechanics: Theory and Experiment

Authors:

David Martin, Gianmarco Spera, Hugues Chaté, Charlie Duclut, Cesare Nardini, Julien Tailleur, and Frédéric Wijland

Abstract:

The nature of the transition to collective motion in assemblies of aligning self-propelled particles remains a long-standing matter of debate. In this article, we focus on dry active matter and show that weak fluctuations suffice to generically turn second-order mean-field transitions into a 'discontinuous' coexistence scenario. Our theory shows how fluctuations induce a density-dependence of the polar-field mass, even when this effect is absent at mean-field level. In turn, this dependency on density triggers a feedback loop between ordering and advection that ultimately leads to an inhomogeneous transition to collective motion and the emergence of inhomogeneous travelling bands. Importantly, we show that such a fluctuation-induced first order transition is present in both metric models, in which particles align with neighbors within a finite distance, and in 'topological' ones, in which alignment is based on more complex constructions of neighbor sets. We compute analytically the noise-induced renormalization of the polar-field mass using stochastic calculus, which we further back up by a one-loop field-theoretical analysis. Finally, we confirm our analytical predictions by numerical simulations of fluctuating hydrodynamics as well as of topological particle models with either k-nearest neighbors or Voronoi alignment.
More details from the publisher
Details from ArXiV

Nematic Torques in Scalar Active Matter: when Fluctuations Favor Polar Order and Persistence

Physical Review Letters

Authors:

Gianmarco Spera, Charlie Duclut, Marc Durand, and Julien Tailleur

Abstract:

We study the impact of nematic alignment on scalar active matter in the disordered phase. We show that nematic torques control the emergent physics of particles interacting via pairwise forces and can either induce or prevent phase separation. The underlying mechanism is a fluctuation-induced renormalization of the mass of the polar field that generically arises from nematic torques. The correlations between the fluctuations of the polar and nematic fields indeed conspire to increase the particle persistence length, contrary to what phenomenological computations predict. This effect is generic and our theory also quantitatively accounts for how nematic torques enhance particle accumulation along confining boundaries and opposes demixing in mixtures of active and passive particles.
More details from the publisher
Details from ArXiV

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